A vaccum is a volume of space that is essentially empty of matter, so that gaseous pressure is much less than standard atmospheric pressure. The root of the word vaccum is the Latin adjective vacuus which means "empty," but space can never be perfectly empty. A perfect vaccum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice, not least because quantum theory predicts that no volume of space is perfectly empty in this way. Physicists often use the term "vaccum" slightly differently. They discuss ideal test results that would occur in a perfect vaccum, which they simply call "vaccum" or "free space" in this context, and use the term partial vaccum to refer to the imperfect vacua realized in practice. The quality of a vaccum is measured by how closely it approaches a perfect vaccum. The residual gas pressure is the primary indicator of quality, and it is most commonly measured in units of torr, even in metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account. Quantum mechanics sets limits on the best possible quality of vaccum. Outer space is a natural high quality vaccum, mostly of much higher quality than what can be created artificially with current technology. Low quality artificial vaccums have been used for suction for millennia. vaccum has been a common topic of philosophical debate since Ancient Greek times, but it was not studied empirically until the 17th century. Experimental techniques were developed following Evangelista Torricelli's theories of atmospheric pressure. vaccum became a valuable industrial tool in the 20th century with the introduction of the light bulb and vaccum tube, and a wide array of vaccum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vaccum on human health, and life forms in general.